US 6989823 B1
A method for reducing phase noise and distortion in a digitally converted image in which only a selected subset of the video frames that represent the image are displayed. The selected subset is comprised of reference video frames, where a new reference frame is captured for display after determining that a pixel of an inbound video frame exceeds a certain threshold of change as compared to a corresponding pixel of a previously captured reference frame. The pixel comparison is performed by comparing corresponding numerical values representing each of the pixel's color values, red, green, and blue, or by comparing corresponding numerical values representing a composite color value. The threshold determination is performed by comparing the absolute value of the difference between each of the corresponding pixel values to a pre-selected change threshold value. After determining that the threshold has been exceeded, the next consecutive inbound frame is captured and stored in a frame buffer memory as a new reference frame. All of the stored reference frames are transmitted from the frame buffer memory for display to a display object in accordance with the display object's frame refresh rate. The threshold value is pre-selected to eliminate the intervening sub-threshold inbound frames causing the phase noise and distortion, which not only improves the quality of the displayed image but also reduces the total number of new frames that must be transmitted to the display object.
1. An apparatus comprising:
a frame conversion unit to convert frames of analog image data to frames of digital image data;
a buffer coupled with the frame conversion to store the frame of digital image data and subsequent converted frames;
a pixel value comparator coupled with the buffer to compare pixel data of the frame of digital image data and pixel data from the subsequent converted frame using horizontal position counter to compare and identify a subsequent converted frame having pixel data that differs from the pixel data of the frame of digital image data by a threshold amount;
a capture switch configured to be set to on when the subsequent converted frame pixel data exceeds the threshold amount and where the capture switch is configured to be set to off when the subsequent converted frame pixel data is below the threshold amount;
a microcontroller configured to capture the subsequent converted frame with pixel data that exceeds the threshold amount for storage to replace the frame of digital image data and display by a display object, and discard frames with pixel data that below the threshold amount.
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1. Field of the Invention
The present invention relates generally to reducing phase noise and distortion in the conversion of an analog video signal to a digital video signal. In particular, the present invention is directed to technology that reduces phase noise and distortion in the digitally converted image by selectively capturing reference frames of the image in accordance with a change threshold.
2. Background Information
Presentations using multimedia projection display systems have become popular for purposes such as sales demonstrations, business meetings, and classroom sessions. In a common mode of operation, multimedia projection display systems receive analog video signals from a multimedia device such as a personal computer (PC). The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted into digital video signals to control a digitally-driven display object, such as a transmissive liquid crystal display (LCD) or digital mirror device (DMD), to form the display images for projection onto a display screen.
Two common types of multimedia projection display systems are LCD projectors and LCD projection panels. An LCD projector includes a transmissive LCD, a light source, and projection optics to form and project display images in the manner described above. An LCD projection panel includes a similar transmissive LCD to form the display image, but operates with a conventional overhead projector (OHP) having a light source and projection optics, to project the display image onto a display screen. Examples of such LCD projectors and LCD projection panels are sold under the respective trademarks LITEPRO and PANELBOOK by In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present application.
Because the LCDs and DMDs used in multimedia projection display systems require digital video signals, the multimedia projection display system normally has an analog to digital (A/D) signal converter for converting the PC-generated analog video signals into a digital format suitable for driving the LCD, DMD or other type of display object.
The PC-generated analog video signal is actually comprised of three analog image data signals, one for each of the primary colors, red, green and blue, and a digital timing signal, which may include a pulsed horizontal synchronizing signal (Hsync) as well as a pulsed vertical synchronizing signal (Vsync), or a composite sync signal. The individual analog image data signals representing the colors are generated from bit data in a memory portion of the PC, using three digital-to-analog (D/A) converters, one for each of the colors, red, green, and blue.
A complete image is typically displayed during a time interval known as a “video frame period.” With reference to
The A/D signal converter is usually combined with a phase-locked loop (PLL), which may comprise a phase comparator, a low-pass loop filter, and a voltage-controlled oscillator (VCO) formed in a loop to generate a feedback signal that locks into the Hsync pulsed horizontal synchronizing signal. In order to generate a selected multiple n of pixel clock pulses for each period of Hsync, a divide-by-n counter is added to the feedback loop between the VCO output and the phase comparator.
An example of a pixel clock waveform 4 is shown in
One problem with the above-described A/D conversion is that it is often imperfect due to errors in the pixel clock sampling of the analog signal. With reference to
Prior art techniques for reducing noise have focused on providing feedback to the pixel clock in order to eliminate the above-described tracking and phase errors. For example, a current projection display system may include an image capture circuit that automatically eliminates phase and tracking errors by monitoring the actual active image width of the analog video data signal measured by the number n of pixel clocks, and adjusting the frequency and phase components of the pixel clock signal until the expected width of the image matches the actual width. A current projection display system may also include an image capture circuit that automatically eliminates phase and tracking errors by monitoring a selected pixel data component at the edge of the central active video region that is horizontally stationary from frame to frame of the analog video data, and automatically iteratively adjusting the pixel clock pulse until a pixel clock pulse is centrally registered with the selected pixel data component. Both of these prior art techniques used in current projection display systems are described in U.S. Pat. Nos. 5,767,916 and 5,805,233, assigned to In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present invention.
The image capture circuit of a current projection display system includes a programmable delay device, a PLL, a divide-by-n-counter, an A/D converter, and an ASIC (Application Specific Integrated Circuit) that contains the image edge or width detection circuitry that provides the feedback necessary to correct the pixel clock.
In operation, the A/D converter samples (reads) the instantaneous voltage value of the analog video data signal at the leading edge of each of the pixel clocks, thereby generating a series of sampled data signal values. The A/D converter then quantizes the sampled values by matching each value to one of a series of preselected voltage amplitude levels, which have corresponding numerical values. The numerical values are represented digitally and coded to establish 8-bit data for each of the colors, red, green, and blue. The three eight-bit color data signals are input through the three respective color data signal channels to the ASIC. A window random access memory (WRAM) is connected between the ASIC and the LCD, DMD, or other display object that ultimately receives the output from the A/D converter. At the display object, the coded color data signals set pixels to blank (black) or to specific activated (non-black) status corresponding to the sampled voltage level. A microcontroller, which is part of the current projection display system CPU, uses the feedback provided by the ASIC to control the programmable delay device and divide-by-n-counter to change the pixel clock settings and eliminate the phase and tracking errors.
Despite the above-described improvements in eliminating phase and tracking errors, current projection display systems are still not optimum. Particularly frustrating is the fact that the causes of the phase noise and distortion may dynamically change as the environment changes. Thus, any given technique for reducing or eliminating the distortion may or may not be effective from one moment to the next.
Another problem is that a digitized video frame representing the complete image displayed during the video frame period must be stored within the WRAM until it is transmitted to the display object. Since the video frames are typically stored in WRAM at a faster rate than they are transmitted to the display object, the WRAM must include enough frame buffer capacity or memory to store a number of video frames at once. For example, each video frame may be stored to the WRAM at the rate of 80 Hz (i.e. 80 frames per second), but may only be output to the display object at the rate of 60 Hz, as dictated by the refresh rate of the particular display object (e.g. the LCD, DMD, etc.).
For even the lowest resolution video display systems, storing video frames in WRAM results in the storage of a significantly large amount of data. Since a video frame represents the complete image, it contains data representing every pixel in the central active video region. The optical state of each pixel in the central active video region, i.e. its color or shade of gray, is described by several bits of data; the exact number of bits depends upon the desired number of colors or shades of gray that are to be displayed. For example, a typical LCD may have 480 rows and 640 columns that intersect to form a matrix of 307,200 pixels corresponding to the pixels in the central active video region. Consequently, if 8 bits are required to specify each of the three colors comprising each pixel, then 921,600 bytes of image data (3 bytes×307,200 pixels), or nearly 1 megabyte, are required to characterize a single digitized video frame.
The problem of having to provide a necessarily large WRAM storage capacity is compounded by the fact that the WRAM is expensive, which adds significantly to the cost of the multimedia projection display system. Additionally, the WRAM takes up a large amount of board space. Moreover, the sheer volume of digitized video frames generated by the multimedia projection display system requires a significantly large transmission bandwidth between the WRAM and the LCD, DMD, or other display object. Large transmission bandwidth requirements present a problem when designing networked presentation projector applications. For example, in a wireless network environment, including wireless networks designed in accordance with the IEE 802.11 Wireless LAN Standard, or the Draft Specification of Bluetooth: A Global Specification for Wireless Connectivity, promulgated by the Bluetooth Special Interest Group, or even in a conventional networked environment where the LCD, DMD, or other display object is capable of providing its own frame memory, it is desirable to reduce the transmission bandwidth requirements where possible to enable the development of networked presentation projector applications.
Accordingly, it is desirable to provide a multimedia projection display system that not only reduces the noise and distortion in the digitally sampled image, but also reduces the transmission bandwidth requirements between the WRAM and the display object.
According to one aspect of the invention, a method is provided in which a digitally converted image is displayed free of phase noise and distortion and using a reduced amount of stored frame memory by displaying only a selected subset of the video frames that represent the digitally converted image. The selected subset is comprised of reference video frames, where a new reference frame is captured for display only after determining that a pixel of an inbound video frame exceeds a certain threshold of change as compared to a corresponding pixel of the previously captured reference frame.
In accordance with other aspects of the present invention, apparatus are provided for carrying out the above and other methods.
The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which:
In the following description various aspects of the present invention, a method and apparatus for reducing phase noise and distortion in a digitally converted image, will be described. Specific details will be set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some or all of the described aspects of the present invention, and with or without some or all of the specific details. In some instances, well known features may be omitted or simplified in order not to obscure the present invention.
Various operations will be described as multiple discrete steps performed in turn in a manner that is most helpful in understanding the present invention. However, the order of description should not be construed as to imply that these operations are necessarily performed in the order they are presented, or even order dependent. Lastly, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.
Devices that generate digitally converted images generally refresh the screen display at rates of 60–80 Hz (60–80 frames per second) or more to provide a quality image and to avoid eye fatigue. However, the human eye perceives only a portion of the frames, approximately one every 16 milliseconds, or about ⅙th of the frames displayed at 60 Hz. Therefore, it is possible to reduce the number of frames sent to the device without impairing the perceived quality of the image. A reduction in the number of frames sent to the device is particularly desirable in a wireless network environment, or where the device is capable of providing its own frame memory, as it reduces the amount of bandwidth necessary to transmit the frame data between the multimedia projection display system and the display object.
The method of the present invention is based in part on the observation that selectively sending new frames only when there is a perceptively significant change in the frame data not only reduces the number of frames sent to the device, thereby reducing bandwidth requirements, but also improves the overall quality of the perceived image by eliminating unnecessary phase noise and distortion in the digitally converted image.
Referring now to
Turning to the signal path to the display object, such as an active matrix LCD 28, shown in
Based on the output of the image edge detection circuit 26 the microcontroller 16 further controls the delay device 18 and the counter 22 to eliminate phase noise and tracking errors. Based on the output of the reference frame capture circuit 27, the microcontroller 16 further controls the capture to frame memory in WRAM 132 of a reference frame from the frames output from the A/D converter 24 so as to eliminate phase noise and image distortion. The number of reference frames captured depends on the type of image being processed and a threshold setting such that the rate of capture of a reference frame is lower when the image is static (e.g. fixed image presentations), and higher when the image is dynamic (e.g. full motion video). The threshold setting is pre-selected to provide for the maximum elimination of phase noise and distortion, while providing a sufficient number of reference frames for proper replication of the digitally converted image at the display object 28. It should be understood that the outputs of the image edge detection circuit 26 and reference frame capture circuit 27 are both independent and complementary such that the reference frame capture circuit 27 may be used alone or in combination with the image edge detection circuit 26 or other methods to improve the quality of the digitally converted image displayed by the display object 28.
As shown in
Turning to the details of the image capture circuit 14, the microcontroller 16 is connected to the delay device 18 by a bus 38, to the counter 22 by a bus 40, and to the ASIC 26 by a bus 42. A mode identification counter 43, which is connected to Hsync and Vsync through conductors 45 and 47, respectively, may be located in the microcontroller or the ASIC. The mode identification counter 43 may also be provided independent of the microcontroller. A preferred microcontroller 16 is model MC6833 1, made by Motorola. The delay device has an input connected to the Hsync conductor 34, and an output connected to the PLL through conductor 44. The preferred delay device is model No. DS10205-25, made by the Dallas Corporation.
As shown in detail in
Referring again to
As shown in detail in
Referring now to
In operation, the analog video signal is digitized in a manner set forth and described in U.S. Pat. Nos. 5,767,916 and 5,805,233. Referring now to
Digitization of the analog video data signals occurs based on the n pixel clocks per line. Referring now to
The PLL works by the phase comparator 46 receiving the Hsync signal from the delay device 18 through conductor 44, and receiving the feedback pulse signal through the feedback loop 52. The phase comparator 46 compares the frequencies of the Hsync and the feedback pulse signal, generating an output voltage that is a measure of their phase difference. If the feedback pulse frequency does not equal the Hsync frequency, the phase difference signal causes the VCO pixel clock frequency to deviate so that the feedback pulse frequency of the counter 22 deviates toward the Hsync frequency.
Referring again to
The A/D converter 24 samples (reads) the instantaneous voltage value of the analog video data signal at the leading edge of each of the pixel clocks, thereby generating a series of sampled data signal values. The A/D converter 24 then quantizes the sampled values by matching each value to one of a series of preselected voltage amplitude levels, which have corresponding numerical values. These numerical values are then represented digitally and coded to establish 8-bit data for each of the colors red, green and blue.
Referring now to
Using the output of the reference frame capture circuit 27, the microcontroller 16 controls the capture of a reference frame from the output of the A/D converter 24. The captured reference frame is stored in WRAM 132 for eventual transmission to the LCD, DMD, or other display object 28. At the display object 28, the coded color data signal sets pixels to blank (black) or to a specific activated (non-black) status corresponding to the sampled voltage level.
Referring now to
Referring now to
Referring again to
The entire process of capturing and comparing is repeated continually throughout the duration of the image conversion. The result is that the number of reference frames captured by the microcontroller 16 for storage in WRAM 132 and eventual display by display object 28 is a subset of the total number of inbound frames output by the A/D converter 24. Moreover, by selectively capturing an inbound frame only after determining that a preceding inbound frame exceeds a certain threshold of change, inbound frames whose pixel values are either static or that represent below-threshold changes that indicate phase noise or distortion can be eliminated.
An example implementation of a method for reducing phase noise and distortion in a digitally converted image based on reference frame capture will now be described. Referring to
Referring now to
The position of each pixel with a given line is indicated by the current value of the HPC 105. The threshold value in this example is pre-set to the value of 5. Lines 1–5 of frame 1 is stored as the first reference frame 175. The pixel values of the lines comprising inbound frame 2 are compared to the values of reference frame 1, however since none of the differences between the corresponding pixel values of the lines comprising frames 1 and 2 exceed the pre-set threshold of 5, no action results. For example, the pixels located in reference frame 1, Line 4, HPC 6 and inbound frame 2, Line 4, HPC 6 result in the pixel value comparison 115 as follows:
Examining the same relative pixel locations in inbound frame 3232, however, yields a different result. This time, the pixels located in reference frame 1, Line 4, HPC 6 and inbound frame 3232, Line 4, HPC 6 result in the pixel value comparison 115 as follows:
As shown, advancing to the next inbound frame 3233 results in the action of storing 175 frame 3233 as the new reference frame against which subsequent pixel value comparisons will be based.
As noted earlier, in one embodiment, the individual RGB values are used as the pixel values by the pixel value comparator 115. Referring now to
Accordingly, a novel method and apparatus is described for reducing phase noise and distortion in a digitally converted image using a captured reference frame. From the foregoing description, those skilled in the art will recognize that many other variations of the present invention are possible. Thus, the present invention is not limited by the details described. Instead, the present invention can be practiced with modifications and alterations within the spirit and scope of the appended claims.